USB only provides 5 VDC, so any USB adapter will have the correct voltage. Now they might not provide enough current. Many phone adapters have less than 1 amp out, which is probably not enough for the Stick. But I'd have to pull my 3600 out of my suitcase to see what it's adapter rated at. I can do that when I get home if someone doesn't offer it here first.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.